Social media marketing effectiveness should be measured by tracking key metrics like engagement, conversions, and traffic rather than just follower numbers. Engagement metrics to track include time spent on sites or applications, bounce rates, membership and interaction rates over time. Conversions like subscriptions, sales, or other goals should also be measured. Additional metrics that indicate brand awareness, loyalty, and virality can provide useful insights. Traffic sources and behavior on your website from social media should be analyzed using tools like Google Analytics.